Temperature sensing in a printhead using piezoelectric actuators

ABSTRACT

Devices and method for performing temperature measurements in a printhead. In one embodiment, a printhead includes at least one row of jetting channels configured to jet droplets of a print fluid using piezoelectric actuators. A drive circuit includes an input voltage generator that applies a step voltage to a piezoelectric actuator of a jetting channel, and an output voltage detector that detects an output voltage across the piezoelectric actuator over time in response to the step voltage. The drive circuit also includes a temperature detector that determines a voltage response to the step voltage at the piezoelectric actuator based on the output voltage over time, and determines a temperature measurement for the piezoelectric actuator based on the voltage response of the piezoelectric actuator.

FIELD OF THE INVENTION

The following disclosure relates to the field of image formation, and inparticular, to printheads and the use of printheads.

BACKGROUND

Image formation is a procedure whereby a digital image is recreated on amedium by propelling droplets of ink or another type of print fluid ontoa medium, such as paper, plastic, a substrate for 3D printing, etc.Image formation is commonly employed in apparatuses, such as printers(e.g., inkjet printer), facsimile machines, copying machines, plottingmachines, multifunction peripherals, etc. The core of a typical jettingapparatus or image forming apparatus is one or more liquid-dropletejection heads (referred to generally herein as “printheads”) havingnozzles that discharge liquid droplets, a mechanism for moving theprinthead and/or the medium in relation to one another, and a controllerthat controls how liquid is discharged from the individual nozzles ofthe printhead onto the medium in the form of pixels.

A typical printhead includes a plurality of nozzles aligned in one ormore rows along a discharge surface of the printhead. Each nozzle ispart of a “jetting channel”, which includes the nozzle, a pressurechamber, and an actuator, such as a piezoelectric actuator. A printheadalso includes a drive circuit that controls when each individual jettingchannel fires based on image data. To jet from a jetting channel, thedrive circuit provides a jetting pulse to the actuator, which causes theactuator to deform a wall of the pressure chamber. The deformation ofthe pressure chamber creates pressure waves within the pressure chamberthat eject a droplet of print fluid (e.g., ink) out of the nozzle.

In an attempt to reduce the size of printheads, the jetting channelswithin the printheads are moved closer together. Also, Drop on Demand(DoD) printing is moving towards higher productivity and quality, whichrequires small droplet sizes ejected at high jetting frequencies. Theprint quality delivered by a printhead depends on ejection or jettingcharacteristics, such as droplet velocity, droplet mass (orvolume/diameter), jetting direction, etc. Temperature of a printhead orthe print fluid in the printhead may affect the jetting characteristics.It is therefore desirable to detect temperature variations across aprinthead to achieve high quality printing. One way to detecttemperature in a printhead is to embed a temperature sensor, such as athermistor or a thermocouple, in a body of the printhead. Although theuse of temperature sensors may be effective in some circumstances, theuse of temperature sensors adds additional parts to printheads that aresubject to failure, and adds additional cost to the printheads.

SUMMARY

Embodiments described herein use readings from the piezoelectricactuators to detect temperature in a printhead. The piezoelectricactuators are used to jet a print fluid from jetting channels of theprinthead. When a piezoelectric actuator is idle, a step voltage isapplied to the piezoelectric actuator, and the voltage response of thepiezoelectric actuator is measured. The voltage response may be mappedto a temperature measurement for the piezoelectric actuator. Therefore,the piezoelectric actuators may be used as temperature sensors within aprinthead. One advantage of using the piezoelectric actuators astemperature sensors is that temperature measurements may be taken atdifferent areas or regions of the printhead to determine a temperaturedistribution across the printhead. Another advantage is that a dedicatedtemperature sensor does not need to be embedded in the printhead.

One embodiment comprises a drive circuit for a printhead that includesone or more rows of jetting channels configured to jet droplets of aprint fluid using piezoelectric actuators. The drive circuit comprisesan input voltage generator configured to apply a step voltage to apiezoelectric actuator of a jetting channel, and an output voltagedetector configured to detect an output voltage across the piezoelectricactuator over time in response to the step voltage. The drive circuitfurther comprises a temperature detector configured to determine avoltage response to the step voltage at the piezoelectric actuator basedon the output voltage over time, and to determine a temperaturemeasurement for the piezoelectric actuator based on the voltage responseof the piezoelectric actuator.

Another embodiment comprises a method of operating a printhead havingone or more rows of jetting channels configured to jet droplets of aprint fluid using piezoelectric actuators. The method comprises applyinga step voltage to a piezoelectric actuator of a jetting channel,detecting an output voltage across the piezoelectric actuator over timein response to the step voltage, determining a voltage response to thestep voltage at the piezoelectric actuator based on the output voltageover time, and determining a temperature measurement for thepiezoelectric actuator based on the voltage response of thepiezoelectric actuator.

Another embodiment comprises a printhead that includes one or more rowsof jetting channels configured to jet droplets of a print fluid. Each ofthe jetting channels comprises a piezoelectric actuator, a pressurechamber, and a nozzle. The printhead also includes a head driverconfigured to apply a step voltage to the piezoelectric actuator of ajetting channel, to detect an output voltage across the piezoelectricactuator over time in response to the step voltage, to determine avoltage response to the step voltage at the piezoelectric actuator basedon the output voltage over time, and to determine a temperaturemeasurement for the piezoelectric actuator based on the voltage responseof the piezoelectric actuator.

The above summary provides a basic understanding of some aspects of thespecification. This summary is not an extensive overview of thespecification. It is intended to neither identify key or criticalelements of the specification nor delineate any scope particularembodiments of the specification, or any scope of the claims. Its solepurpose is to present some concepts of the specification in a simplifiedform as a prelude to the more detailed description that is presentedlater.

DESCRIPTION OF THE DRAWINGS

Some embodiments of the present disclosure are now described, by way ofexample only, and with reference to the accompanying drawings. The samereference number represents the same element or the same type of elementon all drawings.

FIG. 1 is a schematic diagram of a jetting apparatus in an illustrativeembodiment.

FIG. 2 is a perspective view of a printhead in an illustrativeembodiment.

FIGS. 3A and 3B are schematic diagrams of a jetting channel within aprinthead in an illustrative embodiment.

FIG. 4 is a block diagram illustrating a drive circuit for a printheadin an illustrative embodiment.

FIG. 5 illustrates a jetting pulse of a drive waveform for a printhead.

FIG. 6 is a capacitance-temperature graph for a piezoelectric actuatorin an illustrative embodiment.

FIG. 7 is a schematic diagram of a temperature controller in anillustrative embodiment.

FIG. 8 is a flow chart illustrating a method of performing temperaturemeasurements within a printhead in an illustrative embodiment.

FIG. 9 illustrates a step voltage in an illustrative embodiment.

FIG. 10 illustrates a voltage response in an illustrative embodiment.

FIG. 11 is a flow chart illustrating a method of determining atemperature range or a temperature indicator for a piezoelectricactuator in an illustrative embodiment.

FIG. 12 illustrates the response time of a piezoelectric actuator in anillustrative embodiment.

FIG. 13 is a flow chart illustrating a method of determining atemperature of a piezoelectric actuator in an illustrative embodiment.

FIG. 14 is a schematic diagram of a drive circuit in anotherillustrative embodiment.

FIG. 15 is a flow chart illustrating a method of heating a printhead inan illustrative embodiment.

DETAILED DESCRIPTION

The figures and the following description illustrate specific exemplaryembodiments. It will thus be appreciated that those skilled in the artwill be able to devise various arrangements that, although notexplicitly described or shown herein, embody the principles of theembodiments and are included within the scope of the embodiments.Furthermore, any examples described herein are intended to aid inunderstanding the principles of the embodiments, and are to be construedas being without limitation to such specifically recited examples andconditions. As a result, the inventive concept(s) is not limited to thespecific embodiments or examples described below, but by the claims andtheir equivalents.

FIG. 1 is a schematic diagram of a jetting apparatus 100 in anillustrative embodiment. One example of jetting apparatus 100 is aninkjet printer that performs single-pass or multi-pass printing. Jettingapparatus 100 includes a mount mechanism 102 that supports one or moreprintheads 104 above a medium 112. Mount mechanism 102 may comprise acarriage assembly that reciprocates back and forth along a scan line orscan directions for multi-pass printing. Alternatively, mount mechanism102 may be fixed within jetting apparatus 100 for single-pass printing.Printheads 104 are a device, apparatus, or component configured to ejectdroplets 106 of a print fluid, such as ink (e.g., water, solvent, oil,or UV-curable), through a plurality of orifices or nozzles (not visiblein FIG. 1). The droplets 106 ejected from the nozzles of printheads 104are directed toward medium 112. Medium 112 comprises any type ofmaterial upon which ink or another print fluid is applied by aprinthead, such as paper, plastic, card stock, transparent sheets, asubstrate for 3D printing, cloth, etc. Typically, nozzles of printheads104 are arranged in one or more rows so that ejection of print fluidfrom the nozzles causes formation of characters, symbols, images, layersof an object, etc., on medium 112 as printhead 104 and/or medium 112 aremoved relative to one another. Media transport mechanism 114 isconfigured to move medium 112 relative to printheads 104. Jettingapparatus 100 also includes a jetting apparatus controller 122 thatcontrols the overall operation of jetting apparatus 100. Jettingapparatus controller 122 may connect to a data source to receive imagedata, and control each printhead 104 to discharge the print fluid on adesired pixel grid on medium 112.

FIG. 2 is a perspective view of printhead 104 in an illustrativeembodiment. Printhead 104 includes a nozzle plate 202, which representsthe discharge surface of printhead 104. Nozzle plate 202 includes aplurality of nozzles that jet or eject droplets of print fluid, and thenozzles are arranged in rows 210-211. Although two rows 210-211 ofnozzles are illustrated in FIG. 2, printhead 104 may include a singlerow of nozzles, three rows of nozzles, four rows of nozzles, etc.Printhead 104 also includes attachment members 204. Attachment members204 are configured to secure printhead 104 to a jetting apparatus, suchas to mount mechanism 102 as illustrated in FIG. 1. Attachment members204 may include one or more holes 206 so that printhead 104 may bemounted within a jetting apparatus by screws, bolts, pins, etc. Oppositenozzle plate 202 is the side of printhead 104 used for input/output(I/O) of print fluid, electronic signals, etc. This side of printhead100 is referred to as the I/O side 224. I/O side 224 includeselectronics 226 that connect to a controller board through cabling 228,such as a ribbon cable. Electronics 226 control how the nozzles ofprinthead 104 jet droplets in response to control signals provided bythe controller board.

FIG. 3A is a schematic diagram of a row of jetting channels 302 withinprinthead 104 in an illustrative embodiment. Printhead 104 includesmultiple jetting channels 302 that are arranged in a line or row (e.g.,row 210 in FIG. 2) along a length of printhead 104, and each jettingchannel 302 in a row may have a similar configuration as shown in FIG.3A. Each jetting channel 302 includes a piezoelectric actuator 310, apressure chamber 312, and a nozzle 314. FIG. 3B is a schematic diagramof a jetting channel 302 within printhead 104 in an illustrativeembodiment. The view in FIG. 3B is of a cross-section of jetting channel302 across a width of printhead 104. The arrow in FIG. 3B illustrates aflow path of a print fluid within jetting channel 302. The print fluidflows from a supply manifold in printhead 104 and into pressure chamber312 through restrictor 318. Restrictor 318 fluidly connects pressurechamber 312 to a fluid supply, such as a supply manifold, and controlsthe flow of the print fluid into pressure chamber 312. One wall ofpressure chamber 312 is formed with a diaphragm 316 that physicallyinterfaces with piezoelectric actuator 310. Diaphragm 316 may comprise asheet of semi-flexible material that vibrates in response to actuationby piezoelectric actuator 310. The print fluid flows through pressurechamber 312 and out of nozzle 314 in the form of a droplet in responseto actuation by piezoelectric actuator 310. Piezoelectric actuator 310is configured to receive a drive waveform, and to actuate or “fire” inresponse to a jetting pulse on the drive waveform. Firing ofpiezoelectric actuator 310 in jetting channel 302 creates pressure wavesin pressure chamber 312 that cause jetting of a droplet from nozzle 314.

Jetting channel 302 as shown in FIGS. 3A-3B is an example to illustratea basic structure of a jetting channel, such as the actuator, pressurechamber, and nozzle. Other types of jetting channels are also consideredherein. For example, some jetting channels may be a “flow-through” typehaving another restrictor that fluidly connects pressure chamber 312 toa return manifold (not shown) in printhead 104. Some jetting channelsmay have a pressure chamber having a different shape than is illustratedin FIGS. 3A and 3B.

FIG. 4 is a block diagram illustrating a drive circuit 400 for printhead104 in an illustrative embodiment. Drive circuit 400 is an apparatus orassembly of circuits/devices that controls printhead 104 to eject aprint fluid on a medium to form pixels on the medium. Drive circuit 400includes a drive waveform generator 402 configured to generate a drivewaveform 404 (e.g., a trapezoidal waveform), and provide the drivewaveform 404 to printhead 104 as a drive signal for piezoelectricactuators 310. Although not illustrated, drive waveform generator 402may also include an amplifier circuit that amplifies the current ofdrive waveform 404. Drive circuit 400 also includes a control signalgenerator 406 configured to provide one or more control signals 408 toprinthead 104 for selectively applying drive waveform 404 to individualjetting channels 302. Control signal generator 406 may receive imagedata, such as serial data, that specifies non-jetting or jetting byindividual jetting channels 302 for pixels. One example of image data isa bitmap that defines pixel locations and values for each pixellocation. In one embodiment, the image data may include two-bit valuesthat define different grey-scale levels for individual pixels. In thisexample, a value of “00” may define non-jetting for a pixel by a jettingchannel 302. A value of “01”, “10”, and “11” may define jetting of one,two, or three droplets for a pixel by a jetting channel 302,respectively. Control signal generator 406 may process the image data,and generate control signals 408 that include data signals, latchsignals, a serial clock, gating signals, etc.

Drive circuit 400 also includes a head driver 410 coupled topiezoelectric actuators 310. Head driver 410 may be an example ofelectronics 226 of printhead 104 as shown in FIG. 2. Head driver 410 isconfigured to selectively control which jetting channels 302 receive thedrive waveform 404, and time windows when the jetting channels 302receive the drive waveform 404. Head driver 410 may comprise anintegrated circuit that is fabricated on printhead 104. Drive waveformgenerator 402 and control signal generator 406 are illustrated as partof jetting apparatus controller 122, which may couple to printhead 104via a signal transmission cable, such as a flexible flat cable (FFC). Inanother embodiment, drive waveform generator 402 may also be part of anintegrated circuit within printhead 104.

Piezoelectric actuators 310 are the actuating devices for jettingchannels 302 that act to jet a droplet out of a nozzle 314 in responseto a jetting pulse. A piezoelectric actuator 310, for example, convertselectrical energy directly into linear motion. To jet from a jettingchannel 302, one or more jetting pulses of the drive waveform 404 areprovided to a piezoelectric actuator 310. A jetting pulse causes adeformation, physical displacement, or stroke of a piezoelectricactuator 310, which in turn acts to deform a wall of pressure chamber312 (e.g., diaphragm 316). Deformation of the chamber wall generatespressure waves inside pressure chamber 312 that force a droplet fromjetting channel 302 (when specific conditions are met). A standardjetting pulse is therefore able to cause a droplet to be jetted from ajetting channel 302 with the desired properties when the jetting channel302 is at rest.

FIG. 5 illustrates a jetting pulse 500 of a drive waveform for aprinthead. The drive waveform in FIG. 5 is shown as an active-lowsignal, but may be an active-high signal in other embodiments. Jettingpulse 500 has a trapezoidal shape, and may be characterized by thefollowing parameters: fall time, rise time, pulse width, and jettingamplitude. Jetting pulse 500 transitions from a baseline (high) voltage501 to a jetting (low) voltage 502 along a leading edge 504. Thepotential difference between the baseline voltage 501 and the jettingvoltage 502 represents the amplitude of jetting pulse 500, which is apeak amplitude of jetting pulse 500. Jetting pulse 500 then transitionsfrom jetting (low) voltage 502 to baseline (high) voltage 501 along atrailing edge 505. These parameters of jetting pulse 500 can impact thejetting characteristics of the droplets from the printhead (e.g.,droplet velocity and mass). For example, when the amplitude of jettingpulse 500 equals a target jetting amplitude (i.e., the jetting voltage)for a target pulse width, a droplet of a desired velocity and mass isjetted from a jetting channel 302. A standard jetting pulse 500 may beselected for different types of printheads to produce droplets having adesired shape (e.g., spherical), size, velocity, etc.

The following provides an example of jetting a droplet from a jettingchannel using jetting pulse 500, such as from jetting channel 302 inFIGS. 3A-3B. Jetting pulse 500 is initially at the baseline voltage 501,and transitions from the baseline voltage 501 to the jetting voltage502. The leading edge 504 (i.e., the first slope) of jetting pulse 500causes a piezoelectric actuator 310 to displace in a first direction,which enlarges pressure chamber 312 and generates negative pressurewaves within pressure chamber 312. The negative pressure waves propagatewithin pressure chamber 312 and are reflected by structural changes inpressure chamber 312 as positive pressure waves. The trailing edge 505(i.e., the second slope) of jetting pulse 500 causes the piezoelectricactuator 310 to displace in an opposite direction, which reducespressure chamber 312 to its original size and generates another positivepressure wave. When the timing of the trailing edge 505 of jetting pulse500 is appropriate, the positive pressure waves created by thepiezoelectric actuator 310 displacing to reduce the size of pressurechamber 312 will combine with the reflected positive pressure waves toform a combined wave that is large enough to cause a droplet to bejetted from nozzle 314 of jetting channel 302. Therefore, the positivepressure waves generated by the trailing edge 505 of jetting pulse 500acts to amplify the positive pressure waves that reflect within pressurechamber 312 due to the leading edge 504 of jetting pulse 500. Thegeometry of pressure chamber 312 and jetting pulse 500 are designed togenerate a large positive pressure peak at nozzle 314, which drives theprint fluid through nozzle 314.

In the embodiments described herein, drive circuit 400 is enhanced toperform temperature measurements in printhead 104 using thepiezoelectric actuators. A piezoelectric actuator 310 behaves like acapacitor. The capacitance of a piezoelectric actuator 310 may depend onthe area and thickness of the crystal or ceramic, and materialproperties of the crystal or ceramic used in the piezoelectric actuator310. The capacitance of a piezoelectric actuator 310 may also depend onthe temperature of or around the piezoelectric actuator 310. FIG. 6 is acapacitance-temperature graph 600 for a piezoelectric actuator 310 in anillustrative embodiment. The vertical axis is the capacitance of apiezoelectric actuator 310 in picofarad (pF). The horizontal axis is thetemperature of a piezoelectric actuator 310 in degrees Celsius. As isevident from graph 600, the capacitance of a piezoelectric actuator 310has a linear or substantially linear relationship with temperature. Whenthe temperature of a piezoelectric actuator 310 is low (e.g., less than35° C.), the capacitance of the piezoelectric actuator 310 is low (e.g.,less than 1080 pF). When the temperature of a piezoelectric actuator 310is high (e.g., more than 60° C.), the capacitance of the piezoelectricactuator 310 is high (e.g., more than 1150 pF). Thus, the capacitance ofa piezoelectric actuator 310 may be used as an indicator of temperatureat the piezoelectric actuator 310. The temperature of the piezoelectricactuator 310 is indicative of the temperature in printhead 104 at thelocation of the piezoelectric actuator 310 within the printhead 104.Thus, temperature detection of piezoelectric actuators 310 can be usedto determine a temperature distribution across printhead 104.

In FIG. 4, head driver 410 includes a temperature controller 418, whichcomprises circuits, logic, processors, etc., configured to performtemperature measurements at piezoelectric actuators 310 within printhead104. Temperature controller 418 includes an input voltage generator 420,an output voltage detector 422, and a temperature detector 424. Inputvoltage generator 420 is configured to generate a step voltage that isapplied to a piezoelectric actuator 310. Output voltage detector 422 isconfigured to detect an output voltage across the piezoelectric actuator310 over time in response to the step voltage. Temperature detector 424is configured to determine a voltage response of the piezoelectricactuator 310 in response to the step voltage, and determine atemperature measurement based on the voltage response of thepiezoelectric actuator 310. A further description of the operation ofhead driver 410 is described below in FIG. 7.

FIG. 7 is a schematic diagram of temperature controller 418 in anillustrative embodiment. As discussed above, printhead 104 includes aplurality of jetting channels 302, and each jetting channel 302 includesa piezoelectric actuator 310. Because a piezoelectric actuator 310behaves like a capacitor, piezoelectric actuators 310 are illustrated ascapacitors in FIG. 7. Printhead 104, or more particularly, head driver410 of printhead 104, includes a plurality of switching elements 702,which may also be referred to as transmission gates. A switching element702 is associated with an individual jetting channel 302, which meansthat an individual switching element 702 is electrically coupled to apiezoelectric actuator 310 of a jetting channel 302. A switching element702 is configured to selectively enable or disable a conductive path toa piezoelectric actuator 310.

Input voltage generator 420 includes a step voltage source 712 and aresistor 714, although it may include other components in otherembodiments. Step voltage source 712 is configured to generate a stepvoltage (V_(IN)) as is described in further detail below. Output voltagedetector 422 includes a voltage measuring instrument 722 that isconfigured to measure an output voltage across a piezoelectric actuator310, although it may include other components in other embodiments. If,for example, switching element 702 of the jetting channel 302 furthestto the right in FIG. 7 is closed and the other switching elements 702 ofthe other jetting channels 302 are open, then voltage measuringinstrument 722 is able to measure an output voltage across thepiezoelectric actuator 310 furthest to the right in FIG. 7. Voltagemeasuring instrument 722 is able to output a measurement or indicationof the output voltage (V_(OUT)) to another device, such as temperaturedetector 424 in FIG. 4.

FIG. 8 is a flow chart illustrating a method 800 of performingtemperature measurements within a printhead in an illustrativeembodiment. The steps of method 800 will be described with respect tohead driver 410 in FIG. 4, although one skilled in the art willunderstand that the methods described herein may be performed by otherdevices or systems not shown. The steps of the methods described hereinare not all inclusive and may include other steps not shown.

Input voltage generator 420 applies a step voltage to a piezoelectricactuator 310 of a jetting channel 302 (step 802). For example, switchingelement 702 of one of the jetting channels 302 may be closed to enable aconductive path between input voltage generator 420 and thepiezoelectric actuator 310 of that jetting channel 302. With theconductive path enabled, input voltage generator 420 is able to applythe step voltage to the piezoelectric actuator 310. FIG. 9 illustrates astep voltage 900 in an illustrative embodiment. A step voltage, whichmay also be referred to as a step input or a step voltage change,comprises a sudden change in voltage level that happens once. In thisexample, step voltage 900 transitions from a first voltage (V_(IN LOW))to a second voltage (V_(IN HIGH)) over a rise time (T_(RI)). A stepvoltage is distinguished from a periodic signal or a pulse, as thevoltage change in a step voltage occurs once. Because step voltage 900is not a pulse, step voltage 900 will not cause the piezoelectricactuator 310 to jet print fluid from its corresponding nozzle 314 whenapplied to the piezoelectric actuator 310.

In FIG. 8, output voltage detector 422 detects an output voltage(V_(OUT)) across piezoelectric actuator 310 over time in response tostep voltage 900 (step 804). Temperature detector 424 determines avoltage response to the step voltage 900 at the piezoelectric actuator310 based on the output voltage over time (step 806). FIG. 10illustrates a voltage response 1000 in an illustrative embodiment. Stepvoltage 900 is an abrupt change in voltage, and the response of thepiezoelectric actuator 310 may be referred to as a step transientresponse. When the step voltage 900 is applied, current will startflowing around the completed circuit and increase the output voltageacross the piezoelectric actuator 310. Current will continue to flow aslong as there is a voltage difference between the piezoelectric actuator310 and step voltage source 712 across resistor 714 (see FIG. 7). Thus,the output voltage across the piezoelectric actuator 310 will increasefrom a first voltage (V_(OUT LOW)) to a second voltage (V_(OUT HIGH))over a time period until reaching a steady-state voltage.

In FIG. 8, temperature detector 424 determines a temperature measurementfor the piezoelectric actuator 310 based on the voltage response 1000 ofthe piezoelectric actuator 310 (step 808). The temperature measurementmay comprise an actual temperature of the piezoelectric actuator 310(e.g., 60° C.), a temperature range (e.g., between 50° C. and 80° C.,above 50° C., below 50° C., etc.), a temperature indicator or state(e.g., “low”, “high”, etc.), or another type of measurement. Inputvoltage generator 420 forms an RC circuit when electrically coupled to apiezoelectric actuator 310 (see FIG. 7). Thus, the principals of an RCcircuit may be used to analyze the voltage response of a piezoelectricactuator 310, and determine a temperature measurement. For example, thetransient voltage response when charging an RC circuit may becharacterized as:

${{V_{OUT}(t)} = {V_{IN}\left( {1 - e^{- \frac{t}{RC}}} \right)}},$

where R is the resistance of resistor 714, and C is the capacitance ofthe piezoelectric actuator 310.

RC is the time constant in this equation. The lower the RC timeconstant, the faster the voltage response of the piezoelectric actuator310. The higher the RC time constant, the slower the voltage response ofthe piezoelectric actuator 310. If the value of R is constant, the RCtime constant is dependent on the capacitance of the piezoelectricactuator 310. Thus, the lower the capacitance of the piezoelectricactuator 310, the faster the voltage response of the piezoelectricactuator 310. The higher the capacitance of the piezoelectric actuator310, the slower the voltage response of the piezoelectric actuator 310.These principals may be used to determine a temperature measurement forthe piezoelectric actuator 310 based on its voltage response, as thecapacitance of the piezoelectric actuator 310 depends on temperature.

In one embodiment, the temperature measurement may relate thetemperature of a piezoelectric actuator 310 to a temperature range ortemperature indicator. In other words, instead of calculating the exacttemperature of a piezoelectric actuator 310 based on the voltageresponse, the temperature measurement represents a range or indicator ofthe temperature for the piezoelectric actuator 310. FIG. 11 is a flowchart illustrating a method 1100 of determining a temperature range or atemperature indicator for a piezoelectric actuator 310 in anillustrative embodiment. Temperature detector 424 determines a responsetime of the voltage response 1000 (step 1102). FIG. 12 illustrates theresponse time of a piezoelectric actuator 310 in an illustrativeembodiment. FIG. 12 illustrates the voltage response 1000 as shown inFIG. 10. The response time TR is measured between a first voltagethreshold (V_(TH1)) and a second voltage threshold (V_(TH2)). Thevoltage thresholds may be selected as desired. In one embodiment, thefirst voltage threshold may be 10% of the maximum output voltage(V_(OUT HIGH)), and the second voltage threshold may be 90% of themaximum output voltage (V_(OUT HIGH)).

In FIG. 11, temperature detector 424 compares the response time to atime threshold (step 1104). An operator or head designer may define oneor more time thresholds for a printhead. For example, an operator maytest a printhead at different temperatures to determine the responsetime of a piezoelectric actuator, and define one or more time thresholdsthat relate to temperature. The time thresholds may define one or moretemperature ranges or temperature indicators. For instance, assume thata time threshold (T_(THRESH)) is determined for a piezoelectric actuator310 at a threshold temperature (e.g., 50° C.). As described above, alower temperature relates to a lower capacitance of a piezoelectricactuator 310, which relates to a smaller RC constant, which relates to afaster response time. A higher temperature relates to a highercapacitance of a piezoelectric actuator 310, which relates to a largerRC constant, which relates to a slower response time. When the responsetime of a piezoelectric actuator 310 is faster than the time threshold(T_(THRESH)), this is indicative of a temperature lower than thetemperature threshold. When the response time of a piezoelectricactuator 310 is slower than the time threshold (T_(THRESH)), this isindicative of a temperature higher than the temperature threshold.Therefore, the response time may be mapped to temperature ranges ortemperature indicators by defining one or more time thresholds(T_(THRESH)). Temperature detector 424 maps the response time to a firsttemperature range or a first temperature indicator when the responsetime is faster than the time threshold (step 1106). Temperature detector424 maps the response time to a second temperature range or a secondtemperature indicator when the response time is slower than the timethreshold (step 1108). For example, the first temperature range may bebetween 20° C. and 50° C., and the second temperature range may bebetween 50° C. and 80° C. (i.e., the first temperature range is lowerthan the second temperature range). When the temperature measurement isa temperature indicator, the first temperature indicator may be “low” orthe like, and the second temperature indicator may be “high” or thelike. Although one time threshold is discussed above, multiple timethresholds may be defined for multiple temperature ranges (e.g., 20°C.-40° C., 40° C-60° C., 60° C.-80° C.) or multiple temperatureindicators (e.g., low, medium, high).

In another embodiment, temperature detector 424 may determine an actualtemperature of a piezoelectric actuator 310 (within a tolerance). FIG.13 is a flow chart illustrating a method 1300 of determining atemperature of a piezoelectric actuator 310 in an illustrativeembodiment. Temperature detector 424 determines or calculates acapacitance of the piezoelectric actuator 310 based on the voltageresponse 1000 (step 1302). To solve for the capacitance of thepiezoelectric actuator 310, temperature detector 424 may solve for “C”in the equation discussed above at one or more times during the voltageresponse 1000. Alternatively, temperature detector 424 may determine aresponse time T_(R) of the voltage response 1000 (see FIG. 12). Then,temperature detector 424 may solve for “C” in the following equation:

$T_{R} = {C \times R \times \left\{ {{\log \left( {1 - \frac{V_{{TH}\; 1} - V_{OUTLOW}}{V_{OUTHIGH} - V_{OUTLOW}}} \right)} - {\log \left( {1 - \frac{V_{{TH}\; 2} - V_{OUTLOW}}{V_{OUTHIGH} - V_{OUTLOW}}} \right)}} \right\}}$

Temperature detector 424 then maps the capacitance of the piezoelectricactuator 310 to a temperature of the piezoelectric actuator 310 (step1304). For example, temperature detector 424 may access graph 600 (seeFIG. 6) or a similar graph or lookup table that maps capacitance totemperature, and determine a temperature of the piezoelectric actuator310 based on the determined capacitance.

Temperature controller 418 may selectively perform temperaturemeasurements as described above on different piezoelectric actuators 310in printhead 104. For instance, temperature controller 418 mayselectively close the switching element 702 for a jetting channel 302 toperform a temperature measurement on the piezoelectric actuator 310 ofthat jetting channel 302. The temperature measurement of a piezoelectricactuator 310 is performed at a time when the jetting channel 302 is idleor non-jetting. Thus, temperature controller 418 may identify one ormore idle jetting channels, and perform a temperature measurement on theidle jetting channels. Temperature controller 418 may performtemperature measurements on piezoelectric actuators 310 in differentregions of printhead 104 in order to obtain a temperature distributionacross printhead 104. If temperature controller 418 identifies a regionwithin printhead 104 that is below a temperature threshold, then it mayperform operations to heat the printhead locally, as described below.

In one embodiment, temperature controller 418 may control piezoelectricactuators 310 to convert electrical energy into heat. If a region ofprinthead 104 is “cool”, then specialized waveforms are provided topiezoelectric actuators 310 in that region to generate heat withoutcausing those piezoelectric actuators 310 to jet. FIG. 14 is a schematicdiagram of drive circuit 400 in another illustrative embodiment. Inaddition to determining the temperature of piezoelectric actuators,temperature controller 418 is configured to adjust, modify, or changethe temperature across printhead 104. To do so, temperature controller418 may further include a heating controller 1426 and a non-jettingpulse generator 1428. Heating controller 1426 comprises a circuit,firmware, or component that identifies regions in a printhead havingtemperature measurements below a temperature threshold. Non-jettingpulse generator 1428 comprises a circuit, firmware, or component thatgenerates heating waveforms that include one or more non-jetting pulses,and applies the non-jetting pulse(s) to piezoelectric actuators 310under control of heating controller 1426. A “non-jetting pulse” isdefined as a pulse that causes a piezoelectric actuator 310 of a jettingchannel 302 to actuate or fire, but does not cause a droplet to bejetted from the jetting channel 302. For example, the pulse width of anon-jetting pulse may be longer than a jetting pulse so that a dropletis not jetted from the jetting channel 302. In a printhead with aresonant frequency of about 83 kHz, the pulse width of a standardjetting pulse may be about 6 microseconds. At a pulse width of 6microseconds, the pressure waves in a jetting channel 302 combine andpeak at the nozzle 314 to jet a droplet from the nozzle 314. If thenon-jetting pulse has a pulse width between about 12-14 microseconds inan 83 kHz head, then the pressure waves can destructively interfere withone another in the jetting channel 302 so that the combined pressurewave is not large enough to jet a droplet from the jetting channel 302.Non-jetting pulse generator 1428 may be part of drive waveform generator402, which may be programmable to generate jetting pulses andnon-jetting pulses.

FIG. 15 is a flow chart illustrating a method 1500 of heating printhead104 in an illustrative embodiment. There may be uneven jetting patternsin printhead 104 during printing operations, which causes some of thejetting channels 302 to be dormant for a time period. Thus, some regionsof printhead 104 may be cooler than others causing an uneven temperaturedistribution across printhead 104. Additionally, environmentalconditions within a printer may cause an uneven temperature distributionacross printhead 104. Heating controller 1426 identifies a region ofprinthead 104 having one or more temperature measurements below atemperature threshold (step 1502). For example, heating controller 1426may receive temperature measurements from temperature detector 424 forone or more piezoelectric actuators 310 in printhead 104, and identify aregion that is “cool” based on the temperature measurements. Heatingcontroller 1426 identifies one or more piezoelectric actuators 310located in the region (step 1504). Non-jetting pulse generator 1428applies one or more non-jetting pulses to the piezoelectric actuator(s)310 (step 1506) under the control of heating controller 1426. Inresponse to the non-jetting pulses, the piezoelectric actuator(s) 310convert the electric energy of the pulses to heat without jetting adroplet from their corresponding jetting channels 302. Therefore,piezoelectric actuators 310 are able to increase the temperature ofprinthead 104 without actually jetting.

Non-jetting pulse generator 1428 may adjust the amount of heat generatedby piezoelectric actuators 310 so that a target heat is reached. Forexample, non-jetting pulse generator 1428 may increase the number ofnon-jetting pulses sent to piezoelectric actuators 310 to increase theheat generated by piezoelectric actuators 310, or may decrease thenumber of non-jetting pulses sent to piezoelectric actuators 310 todecrease the heat generated by piezoelectric actuators 310. Non-jettingpulse generator 1428 may additionally or alternatively increase theamplitude of the non-jetting pulses to increase the heat generated bypiezoelectric actuators 310, or may decrease the amplitude of thenon-jetting pulses to decrease the heat generated by piezoelectricactuators 310. Non-jetting pulse generator 1428 is able to selectivelyprovide the non-jetting pulses to piezoelectric actuators 310 inprinthead 104 to change the temperature across printhead 104.

Any of the various components shown in the figures or described hereinmay be implemented as hardware, software, firmware, or some combinationof these. For example, a component may be implemented as dedicatedhardware. Dedicated hardware elements may be referred to as“processors”, “controllers”, or some similar terminology. When providedby a processor, the functions may be provided by a single dedicatedprocessor, by a single shared processor, or by a plurality of individualprocessors, some of which may be shared. Moreover, explicit use of theterm “processor” or “controller” should not be construed to referexclusively to hardware capable of executing software, and mayimplicitly include, without limitation, digital signal processor (DSP)hardware, a network processor, application specific integrated circuit(ASIC) or other circuitry, field programmable gate array (FPGA), readonly memory (ROM) for storing software, random access memory (RAM),non-volatile storage, logic, or some other physical hardware componentor module.

Also, a component may be implemented as instructions executable by aprocessor or a computer to perform the functions of the component. Someexamples of instructions are software, program code, and firmware. Theinstructions are operational when executed by the processor to directthe processor to perform the functions of the element. The instructionsmay be stored on storage devices that are readable by the processor.Some examples of the storage devices are digital or solid-statememories, magnetic storage media such as a magnetic disks and magnetictapes, hard drives, or optically readable digital data storage media.

Although specific embodiments were described herein, the scope of theinvention is not limited to those specific embodiments. The scope of theinvention is defined by the following claims and any equivalentsthereof.

1. A drive circuit for a printhead comprising at least one row ofjetting channels configured to jet droplets of a print fluid usingpiezoelectric actuators, the drive circuit comprising: an input voltagegenerator configured to apply a step voltage to a piezoelectric actuatorof a jetting channel; an output voltage detector configured to detect anoutput voltage across the piezoelectric actuator over time in responseto the step voltage; and a temperature detector configured to determinea voltage response to the step voltage at the piezoelectric actuatorbased on the output voltage over time, and to determine a temperaturemeasurement for the piezoelectric actuator based on the voltage responseof the piezoelectric actuator.
 2. The drive circuit of claim 1 wherein:the temperature detector is configured to determine a response time ofthe voltage response, to compare the response time to a time threshold,to map the response time to a first temperature range when the responsetime is faster than the time threshold, and to map the response time toa second temperature range when the response time is slower than thetime threshold; and the first temperature range is lower than the secondtemperature range.
 3. The drive circuit of claim 1 wherein: thetemperature detector is configured to determine a response time of thevoltage response, to compare the response time to a time threshold, tomap the response time to a first temperature indicator when the responsetime is faster than the time threshold, and to map the response time toa second temperature indicator when the response time is slower than thetime threshold.
 4. The drive circuit of claim 1 wherein: the temperaturedetector is configured to determine a capacitance of the piezoelectricactuator based on the voltage response, and to map the capacitance ofthe piezoelectric actuator to a temperature of the piezoelectricactuator.
 5. The drive circuit of claim 1 further comprising: a heatingcontroller configured to identify a region of the printhead having oneor more temperature measurements below a temperature threshold, and toidentify at least one of the piezoelectric actuators located in theregion; and a non-jetting pulse generator configured to apply at leastone non-jetting pulse to the at least one of the piezoelectric actuatorslocated in the region to generate heat.
 6. The drive circuit of claim 5wherein: the at least one non-jetting pulse has a pulse width that islonger than a jetting pulse used to jet.
 7. The drive circuit of claim 5wherein: the non-jetting pulse generator is configured to increase anumber of non-jetting pulses applied to the at least one of thepiezoelectric actuators to increase the heat generated by the at leastone of the piezoelectric actuators in the region, and to decrease thenumber of the non-jetting pulses applied to the at least one of thepiezoelectric actuators to decrease the heat generated by thepiezoelectric actuator in the region.
 8. The drive circuit of claim 5wherein: the non-jetting pulse generator is configured to increase anamplitude of the at least one non-jetting pulse to increase the heatgenerated by the at least one of the piezoelectric actuators in theregion, and to decrease the amplitude of the at least one non-jettingpulse to decrease the heat generated by the at least one of thepiezoelectric actuators in the region.
 9. A method of operating aprinthead having at least one row of jetting channels configured to jetdroplets of a print fluid using piezoelectric actuators, the methodcomprising: applying a step voltage to a piezoelectric actuator of ajetting channel; detecting an output voltage across the piezoelectricactuator over time in response to the step voltage; determining avoltage response to the step voltage at the piezoelectric actuator basedon the output voltage over time; and determining a temperaturemeasurement for the piezoelectric actuator based on the voltage responseof the piezoelectric actuator.
 10. The method of claim 9 whereindetermining the temperature measurement for the piezoelectric actuatorcomprises: determining a response time of the voltage response;comparing the response time to a time threshold; mapping the responsetime to a first temperature range when the response time is faster thanthe time threshold; and mapping the response time to a secondtemperature range when the response time is slower than the timethreshold; wherein the first temperature range is lower than the secondtemperature range.
 11. The method of claim 9 wherein determining thetemperature measurement for the piezoelectric actuator comprises:determining a response time of the voltage response; comparing theresponse time to a time threshold; mapping the response time to a firsttemperature indicator when the response time is faster than the timethreshold; and mapping the response time to a second temperatureindicator when the response time is slower than the time threshold. 12.The method of claim 9 wherein determining the temperature measurementfor the piezoelectric actuator comprises: determining a capacitance ofthe piezoelectric actuator based on the voltage response; and mappingthe capacitance of the piezoelectric actuator to a temperature of thepiezoelectric actuator.
 13. The method of claim 9 further comprising:identifying a region of the printhead having one or more temperaturemeasurements below a temperature threshold; identifying at least one ofthe piezoelectric actuators located in the region; and applying at leastone non-jetting pulse to the at least one of the piezoelectric actuatorslocated in the region to generate heat.
 14. The method of claim 13wherein: the at least one non-jetting pulse has a pulse width that islonger than a jetting pulse used to jet.
 15. The method of claim 13wherein applying at least one non jetting pulse to the at least one ofthe piezoelectric actuators comprises: increasing a number ofnon-jetting pulses applied to the at least one of the piezoelectricactuators to increase the heat generated by the at least one of thepiezoelectric actuators in the region; and decreasing the number of thenon-jetting pulses applied to the at least one of the piezoelectricactuators to decrease the heat generated by the piezoelectric actuatorin the region.
 16. The method of claim 13 wherein applying at least onenon jetting pulse to the at least one of the piezoelectric actuatorscomprises: increasing an amplitude of the at least one non-jetting pulseto increase the heat generated by the at least one of the piezoelectricactuators in the region; and decreasing the amplitude of the at leastone non-jetting pulse to decrease the heat generated by the at least oneof the piezoelectric actuators in the region.
 17. A printheadcomprising: at least one row of jetting channels configured to jetdroplets of a print fluid, wherein each of the jetting channelscomprises a piezoelectric actuator, a pressure chamber, and a nozzle;and a head driver configured to apply a step voltage to thepiezoelectric actuator of a jetting channel, to detect an output voltageacross the piezoelectric actuator over time in response to the stepvoltage, to determine a voltage response to the step voltage at thepiezoelectric actuator based on the output voltage over time, and todetermine a temperature measurement for the piezoelectric actuator basedon the voltage response of the piezoelectric actuator.
 18. The printheadof claim 17 wherein: the head driver is configured to determine aresponse time of the voltage response, to compare the response time to atime threshold, to map the response time to a first temperature rangewhen the response time is faster than the time threshold, and to map theresponse time to a second temperature range when the response time isslower than the time threshold; and the first temperature range is lowerthan the second temperature range.
 19. The printhead of claim 17wherein: the head driver is configured to determine a response time ofthe voltage response, to compare the response time to a time threshold,to map the response time to a first temperature indicator when theresponse time is faster than the time threshold, and to map the responsetime to a second temperature indicator when the response time is slowerthan the time threshold.
 20. The printhead of claim 17 wherein: the headdriver is configured to determine a capacitance of the piezoelectricactuator based on the voltage response, and to map the capacitance ofthe piezoelectric actuator to a temperature of the piezoelectricactuator.